The present invention relates to pulse duration amplifiers and, more particularly, to amplifiers having means for minimizing distortion.
Pulse duration modulation (PDM) is frequently employed in power amplifiers to achieve efficiency of operation. In such amplifiers, an input signal, such as an audio signal, is used to pulse modulate a carrier signal, and the resulting PDM signal is amplified. The amplified PDM signal is then filtered to recover a modulated signal corresponding to an amplified version of the input audio signal.
A polyphase PDM amplifier is disclosed in U.S. Pat. No. 4,164,714 to H. Swanson and assigned to the assignee herein. This patent discloses a system wherein an amplified signal is formed by combining the outputs of plural parallel-connected PDM amplifiers. All of the amplifiers respond to a common input signal, but have differently phased carrier signals. Polyphase PDM amplifiers of this type enjoy various advantages over earlier, single phase PDM amplifiers. For example, amplifier elements employed in polyphase PDM amplifiers operate at relatively low switching frequencies and power levels, permitting semi-conductor devices to be used as the active amplifier elements instead of vacuum tube devices.
The switching frequencies at which the PDM amplifiers operate serve to drive a switching device such as a semi-conductor to generate a train of squarewave pulses of fixed magnitude but variable pulse width in dependence on the on-off time durations of the semi-conductor switching device. It has been found that the amplitude of the amplified PDM signal may change in magnitude, resulting from variations in amplifier current or amplifier drive voltage or variations in the power supply voltage.
Circuits are known in the prior art for minimizing signal distortion in amplifier systems employed in an AM transmitter. The U.S. Pat. Nos. to H. Swanson et al. 4,737,731 and D. H. Covill 4,605,910 provide examples of such circuits. Each of these patents discloses circuitry for minimizing distortion resulting from variations in the DC power supply. In each case, a feed forward technique is employed in which a sample of the input DC voltage signal is obtained and is combined with the input audio signal to compensate for variations in the magnitude of the DC supply voltage prior to supplying the signal to the amplification stages of the transmitter. This feed forward technique, therefore, does not compensate for additional noise resulting as from noisy switches in the amplifier stages.
The U.S. Pat. No. 3,943,446 to D. L. Quidort discloses a system for controlling the output power of a broadcast transmitter employing a pulse width modulator having a triangular wave which is compared with an audio input. The output power is monitored and, in turn, controlled by varying the amplitude of the triangular wave relative to the audio input.